Author to whom all correspondence should be addressed. e-mail: abrams@zoo.utoronto.ca

ABSTRACT

Here, we use some simple models to investigate the impact of between-prey competition on the nature of the indirect evolutionary interaction between prey species via shared predators. Sympatry with a second prey species is likely to alter the population size of the shared predator population(s) and thereby affect the adaptive evolution of anti-predator (vulnerability) traits in the first prey species. Such predation-driven character shift has been investigated previously in models that either assumed no resource-based competition between prey species, or complete overlap in resource use by prey. This study examines the intermediate situation in which prey species compete, but are able to co-exist in the absence of the predator. This scenario is capable of producing either of the two most common responses observed in the limiting cases: parallel change in anti-predator characteristics or divergent change in those characteristics. Parallel change characterizes situations with relatively low competition, and those with high competition when the asymmetry in competition coefficients is less extreme than the asymmetry in the ratio of values of the two prey to the predator. Divergent change occurs with high competition or with moderate competition given sufficient asymmetry in competition coefficients. The factors determining the magnitude of character shifts are explored. Potential examples of such evolutionary responses to sympatry with additional prey species are discussed.

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